Notices add a lot to the game world when they are used well. They are as credible as you'll make them, given that you show a level of respect for those that post them. Try to be sober about them and only post if it is asked, or if it adds a relevance to the in-character element.
Here's a few guidelines from the rumors' sections, some can apply here. The one I'd like to highlight is #8.
1 ) Don't use an RP thread to promote Self-Grata RP.
2 ) Don't use them if it is to post how you killed 4 guards or the entire garrison, specially if not having the backing of a DM.
3 ) Don't use them to mention how you killed a Key NPC on the module - Key NPC's need to be respected if we want to maintain a certain level of realism to persistent RP - Unless you've got DM backing.
4 ) If you post about a gossip, its just that a gossip and should be considered as such, try to post gossips from an objective point of view, not your character's, try to give a feel and see things from another perspective - try the Natives for example or different views - not always the favoring me, myself and I view.
5 ) Be mindful of events to post about that occur in the city or a facility inside the city. Posting about how you slaughtered everyone inside the Blue Water inn, for example could have serious repercussions on the world - Nobles wouldn't attend it, guards would be on the lookout etc, try to post about those things if a DM has watched you and if the RP behind it has something more than because I can to it.
6 ) Try to keep away from the over-the-top: posting about how your character slaughtered 343409 Calibans in the sewers and stuff like that.
7 ) Main action of the RP should happen in game not the forums, initiate the RP in game, give it a twist on the forums, not the other way around.
8 ) Avoid posts such as: [Armand thinks to himself] ''OOooooh Ahhhh! This is certainly a good thing for my plans of world dominance''. [Cackles as he watches the sign] - in short, if you don't emote thoughts, avoid: [Thinks to himself or Passes by and spots the sign - walks away and chuckles!] - its completely irrelevant and contributes in nothing to the persistent world - aside upping your post count, appears as if you are ridiculing someone OOC'ly for an IC post which most likely will reflect poorly on your person and upset the original poster.
9 ) Please post just from the perspective of your character. Don't post a general reply that isn't from one of your characters. Don't post any reactions from the populace or NPCs of the world. In general don't post any reactions from creatures, that could be DM possessed. We DMs will post the reaction of the environment or populace etc.